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Wp7, C# Navigate when download is completed in viewModel
private void DownloadItems()
    {
        string key = this.User.Key;
        WebClient wc = new WebClient();
        wc.DownloadStringCompleted += callback;
        wc.DownloadStringAsync(new Uri("http://localhost/items?key=" + key)); //JSON
    }

private void callback(object sender, DownloadStringCompletedEventArgs e)
    {
        if (e.Error == null)
        {
            List<ItemViewModel> col = Deserialize_ItemViewModel(e.Result); // deserialize JSON to List<ItemViewModel>
            this.Items = new ObservableCollection<ItemViewModel>(col);
            ItemDB.Sponsors.InsertAllOnSubmit(col);
            ItemDB.SubmitChanges();
            this.IsDataLoaded = true;
            // ???
        }
    }

public class VisibilityConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter,
               CultureInfo culture)
    {
        if (value != null && value is bool && parameter != null)
        {
            var bValue = (bool) value;
            var visibility = (Visibility)Enum.Parse(
            typeof (Visibility), parameter.ToString(),true);
            if (bValue) return visibility;
            return visibility ==
            Visibility.Visible ? Visibility.Collapsed : Visibility.Visible;
        }

        return null;
    }

    public object ConvertBack(object value, Type targetType, object parameter,
                   CultureInfo culture)
    {
         throw new NotImplementedException();
    }
}

Visibility="{Binding IsDownloading, Converter={StaticResource VisibilityConverter}, ConverterParameter=Visible}"

Dispatcher.BeginInvoke(delegate

{

 NavigationService.Navigate(new Uri("/Folder/pagename.xaml", UriKind.Relative));

});